Stressed out kitty losing weight

I have a 2 year old cat named Ella who has been my companion since she was a baby. It's just the two of us in a one bedroom apartment so we have a very close bond. I have gone out of town here and there during that time and for the most part she has done well. But, I just returned from a trip today (I was gone 20 days which is the most I have ever been gone) and immediately noticed she has lost weight. Not enough to call the vet, but enough to know she wasn't eating as much as she normally does in my absence. Previously she was at the high end of her healthy weight (just over 9.5 pounds). It's hard to gauge accurately as I don't have a big scale but I would guess she's lost about a half a pound or maybe a pound. I am leaving again in two weeks for another 20 days (these lengthly trips are not characteristic for my schedule and it's just happenstance that they are so close together). 

When I am gone my best friend takes care of her. Ella knows him very well and is very comfortable with him. He comes by once a day for at least an hour to fill up her dishes, clean her box, and give her attention. Then, he spends the night every 3rd day so she has someone to someone to snuggle with. He rotates her toys so she gets a little variety (including a few new ones) to try and cut the loneliness/boredom. He did say she threw up her food a few times but she does that occasionally. I don't think there is anything medically wrong with her (she just had her wellness check just before I left) or else I would not hesitate to take her to the vet. I really believe her vomiting and weight loss are purely stress related. 

I know she would do better if she had a companion and I have planned for some time to add a new addition once these trips are over (I didn't want to add a new cat and risk not having enough time for proper integration). My question is, does anyone have any suggestions of other things I could try in the meantime that may lessen her stress? I have used a feliway calming diffuser with her before but didn't see a noticeable difference in her daily high strung-ness. 

Any suggestions would be appreciated! 

How much weight did she lose?
I am one that likes to be on the safer side.....I would definitely be taking her to the vet before your next trip for a complete Blood panel.... I would do that rather quickly, as if there is something wrong, you want to take care of it before you go away.

When you go, put some Feliway plug-ins on the wall, and also give her some Rescue Remedy (the pet version) - if she doesn't eat wet food, she can put it on the water.

Tell your friend to measure the amount of food she puts in every day, and the amount left the next day, so she knows exactly how much she is eating.

It is common for kitties to not eat well for a day or two when they are stressed because of a trip like that, but not for long periods of time - I would be concerned with the weight loss.

Like you I normally tend to err on the side of caution. I will call the vet on Monday and see what his take on it is and if he thinks I should bring her in. I would be more likely to if she hadn't been seen so recently and checked out 100% fine. 

Measuring her food is a good idea- I'll start doing that now to get an idea of what her average intake should be. She free feeds because she's such a nibbler and never really stops and eats a full meal. I know it takes her 3 days to finish a whole bowl but it's a good idea to have a more exact measurement for when I am gone. And I will try her on the rescue remedy- I have heard good things about it. 

She's grazed several times since I've been home so I know she's not on a hunger strike. And, my friend works in rescue with me so I know he was keeping a close eye on her and knows what to watch for- we have both had experiences with fosters not eating who went south fast. He said she ate a little bit most of the days and all of the nights he was here. I think it's more she's not eating when left alone.

Even if there is a medical issue that needs to be tended to I know my being gone is hard on her and just want to do everything I can to make her as comfortable as possible. I would love to take her with me and I would but I think the stress of having to fly and being somewhere new would cause more damage than being left at home does. 

Keep the ideas coming!